Temporary fix to clear TrackerHitAssociator maps each event

[wmtan]

The change to TrackerHitAssociator to support the consumes interface was done in a suboptimal way that, among other things, did not release the PSimHit maps until the beginning of the next event, causing memory issues. This PR is a simple temporary fix to clear the maps at the proper time.
This fix was requested by David Lange.
Since my tests of this PR are still in progress, it would be prudent to test this PR before merging.
NOTE: The consumes interface for TrackerHitAssociator will later be re-implemented in a more robust and less error-prone way very soon.

[Dr15Jones]

please test

[cmsbuild]

The tests are being triggered in jenkins.

[cmsbuild]

A new Pull Request was created by @wmtan for CMSSW_7_5_X.

Temporary fix to clear TrackerHitAssociator maps each event

It involves the following packages:

SimTracker/TrackerHitAssociation
Validation/GlobalRecHits
Validation/RecoTrack
Validation/TrackerRecHits

@civanch, @nclopezo, @danduggan, @mdhildreth, @cmsbuild, @deguio can you please review it and eventually sign? Thanks.
@makortel, @GiacomoSguazzoni, @rovere, @VinInn, @appeltel, @wmtford, @cerati, @threus, @dgulhan, @venturia this is something you requested to watch as well.
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@nclopezo you are the release manager for this.
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[cmsbuild]

+1
Tested at: 9786875
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8990/45/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-8990/45/summary.html